1. Field of the Invention
This invention relates generally to metallurgical processes, and more particularly to a process for the recovery of alumina from fly ash.
2. Description of Prior Art
It has been known for many years that the fly ash from coal burning power plants contained relatively high amounts of alumina; on the order of thirty percent by weight of the fly ash being alumina. The fly ash, being predominately vitrified matter, is a very hard substance, in the form of microscopic globules of various sizes which are relatively impervious to most chemical reactions. SZEPESI, U.S. Pat. No. 4,048,285 describes a process for recovery of alumina from fly ash by mixing a prescribed amount of CaO or the equivalent amount of Ca(OH).sub.2 for the amount alumina in the dry fly ash, and utilizing a steam treatment at 80.degree.-100.degree. C., then dissolving in nitric acid for one hour, followed by filtration and precipition with ammonia. SZEPESI reports that 95% of the Al.sub.2 O.sub.3 content of the fly ash can be obtained as iron free alumina.
MITCHELL, U.S. Pat. No. 3,393,975 discloses a process for recovery of alumina from fly ash utilizing magnetic separation to remove a magnetic iron fraction, and subjecting the non-magnetic alumina containing fraction to continuous counter current leaching in sulfuric acid. This process does not teach the use of calcining for processing fly ash, but does teach calcining if treating flint clay. The calcining of flint clay is conducted at 750.degree. C. for thirty minutes.
Neither MITCHELL nor SZEPESI teach the high temperature calcining of the present invention nor the acid curing or hydrothermic shock treatment in the sulfuric acid treatment.